The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply.
Alternative sources for developing combustible liquid fuel are desirable. An abundant resource is natural gas. The conversion of natural gas to combustible liquid fuel typically involves a first step of converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. Fischer-Tropsch synthesis is a known means for converting syngas to higher molecular weight hydrocarbon products. Fischer-Tropsch diesel has a very high cetane number and is effective in blends with conventional diesel to reduce NOx and particulates from diesel engines, allowing them to meet stricter emissions standards.
Fischer-Tropsch synthesis is often performed under conditions which produce a large quantity of C21+ wax, also referred to herein as “Fischer-Tropsch wax,” which must be hydroprocessed to provide distillate fuels. Often, the wax is hydrocracked to reduce the chain length, and then hydrotreated to reduce oxygenates and olefins to paraffins. Hydrocracking tends to reduce the chain length of all of the hydrocarbons in the feed. When the feed includes hydrocarbons that are already in a desired range, for example, the distillate fuel range, hydrocracking of these hydrocarbons is undesirable.
As disclosed in co-pending U.S. patent application Ser. No. 12/343,534, incorporated in its entirety by reference, a hybrid Fischer-Tropsch catalyst, also referred to herein as a hybrid synthesis gas conversion catalyst, is described which is capable of converting synthesis gas to a hydrocarbon mixture free of solid wax. One advantage of a process employing this catalyst is that the absence of a solid wax phase eliminates the need for separating, and hydrotreating and/or hydrocracking a waxy product in a separate reactor. As such the hydrocarbon product resulting from this improved process can, in theory, be blended with crude oil.
In practice, however, Fischer-Tropsch synthesis produces a large percentage of olefinic hydrocarbons. An olefinic hydrocarbon is defined as a hydrocarbon in which one or more double bonds exist within the molecule. Olefinic, or unsaturated, hydrocarbons have the potential to be disruptive to refining processes, creating problems including crude heater and preheat train fouling, storage instability and gum deposits. Furthermore, the hydrogenation of olefins, apart from diene saturation, is not practiced in crude oil refining. For this reason, synthetic hydrocarbon mixtures must be treated so as to substantially remove unsaturated hydrocarbons before being blended into crude oil.
It would be desirable to have a means for converting a synthesis gas to a hydrocarbon mixture free of solid wax with a low percentage of olefins.